On Mon, 9 Oct 2000 07:44:53 +0100 Peter Chadwick
<Peter_Chadwick@mitel.com> writes:
>
>My screen supply is the old fashioned approach with a 5R4 rectifier.
Choke input
>with negative lead filtering, and a 6X5 to rectify the ripple across the
choke,
>thus providing bias.
Let's take up a (silicon) diode collection for Peter. :-)
>
>Bias supply regulator is a shunt regulator, using an MJE340, and a
couple of NPN
>transistors of some sort as an long tailed pair amplifier driving the
MJE340.
>Screen regulator is a 6L6G driven by a 6SJ7, with a VR150 reference.
And some HV FET's too!
>Works fine for 4-400s or 4-250's. They don't draw negative screen
current, at
>least not that I've ever seen.
My experience, too.
>I have a 6 volt 15Amp switcher. It's very quiet electrically, and the
plan is
>to use it for the 4CX1000 amp. However, it definitely won't start up on
cold
>heaters, and of course, the thermal time constant of the heater cathode
assembly
>makes the electrolytic dodge a bit difficult. I'm going to use some PNP
>germanium pass transistors in a constant current source for the
filament.
>Germanium because I have them, the beta in the ones I have doesn't
collapse as
>quickly at high collector current as 2N3055s, and the lower Vce sat
means that
>with the switcher delivering 6.8 volts, there's still enough headroom to
>guarantee constant current.
Have you tried it on a cold tube? My experience is that the modern
supplies have a soft enough start that it works o.k.
If not, does your SPS have remote voltage sensing?
If so, I offer this technique I employed long ago in my 4CX1500B amp.
I added a capacitor (w/ a resistor in parallel) in series with the remote
voltage sensing terminals and precharged it from the 5V logic supply.
This fooled the supply into thinking (yes, SPS's do think... strangely)
it had 6V (the heater rating) on the output when it was only outputting
1V, until the capacitor discharged, making for a nice soft start ramp.
73,
Marv WC6W
*
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